Silicon nitride has generated considerable interest as a ceramic material because of its high thermal and oxidative stability and extreme hardness. Other advantageous properties include low electrical conductivity, low coefficient of thermal expansion, excellent thermal shock and creep resistance, high strength at elevated temperatures and excellent corrosion resistance.
One method for obtaining silicon nitride-containing ceramic materials is the pyrolysis of polyorganosilazanes. Seyferth et al. (U.S. Pat. No. 4,482,669) describe the cross-linking of the ammonolysis product of an organodihalosilane in the presence of a basic catalyst such as an alkali metal amide to form a polysilazane ceramic precursor. This material is especially useful as a binder for ceramic powders. Burns (U.S. Pat. No. 4,774,312) describes a process for preparing a crosslinkable polydisilacyclobutasilazane ceramic precursor by reacting chlorodisilacyclobutane with cyclic silazanes. A catalyst such as a metal hydride or metal amide, e.g., lithium diethylamide, can be used for the cross-linking reaction. The preparation of polysilazane ceramic precursors is also described, for example, in U.S. Pat. Nos. 4,689,252 (Lebrun and Porte); 4,612,383 (Laine and Blum); 4,675,424 (King et al.) and 4,722,988 (Porte and Lebrun).
In general, the above methods are deficient in that it is difficult or impossible to control the viscosities of the polysilazanes so that they are suitable for the intended end use of the polymer. For example, low viscosities are desirable for polymers used to produce thin films or to infiltrate porous ceramic bodies, and high viscosities are desirable for making fibers.